Vesicle transport is the general process in eukaryotic cells by which proteins synthesized in the endoplasmic reticulum (ER) are transported via the Golgi network to the various compartments in the cell where they will function. Other proteins are transported by the cell surface where this process may be secreted (exocytosis). Such proteins include membrane bound receptors or other membrane proteins, neurotransmitters, hormones, and digestive enzymes. The transport process uses a series of transport vesicles that shuttle a protein from one membrane-bound compartment (donor compartment) to another (acceptor compartment) until the protein reaches its proper destination (Rothman, J. E and Wieland, F. T. et al. (1996) 727:227-33).
The process begins with the budding of a vesicle out of the donor membrane. The vesicle contains the protein to be transported and is surrounded by a protective coat made up of protein subunits recruited from the cytosol. The initial budding process and coating processes are controlled by a cytosolic GTP-binding protein, either SAR or ARF. When GTP binds and activates SAR, it binds to the donor membrane and initiates the vesicle assembly process. The coated vesicle containing the GTP-SAR complex detaches from the donor compartment and is transported through the cytosol. During the transport process, the SAR-bound GTP is hydrolyzed to GDP, and the inactivated SAR dissociates from the transport vesicle. At this point, the protective coat becomes unstable and dissociates from the enclosed vesicle. The uncoated vesicle is recognized by its acceptor compartment through exposed surface identifiers (v-SNAREs) which bind with corresponding molecules on the acceptor compartment membrane (t-SNAREs). The transport process ends when the vesicle fuses with the target membrane.
The fusion of the transport vesicle with the acceptor compartment membrane follows the initial binding (or docking) of the two compartments and involves the formation of a complex between v-SNARE, t-SNARE, and certain other proteins recruited from the cytosol. Many of these other proteins have been identified although their exact functions in the fusion complex remain uncertain (Tellam, J. T. et al. (1995) J. Biol. Chem. 270:5857-63; Hata, Y. and Sudhof, T. C. (1995) J. Biol. Chem. 270:13022-28). N-ethylmaleimide sensitive factor (NSF) and soluble NSF-attachment protein (SNAP) are two such proteins that are conserved from yeast to man and function in most intracellular membrane fusion reactions. Sec1 represents a family of yeast proteins that function at many different stages in the secretory pathway including membrane fusion. Recently, mammalian homologs of Sec1, called Munc-18 proteins, have been identified (Katagiri, H. et al. (1995) J. Biol. Chem. 270:4963-66; Hata et al. supra). Although Munc-18-1 and Munc-18a were originally found in neural tissue, other isoforms such as Munc-18-2, Munc-18b, and -18c are ubiquitously expressed . Munc-18 proteins specifically bind to a family of t-SNARE proteins known as syntaxins. Like Munc-18, different isoforms of syntaxin are found in different tissues and show specific binding to different Munc-18 isoforms (Hata et al. supra).
Although there is no functional data concerning the role of Munc-18 proteins in vesicle transport, mutations in the gene product of a highly related protein from Caenorhabditis elegans, unc-18, results in accumulation of acetylcholine containing secretory vesicles and abnormalities in development of the C. elegans nervous system (Tellarn et al. supra). Specific functional motifs have yet to be identified in Munc-18 and other related syntaxin-binding proteins. However, studies with various truncated forms of Munc-18 indicate that the entire sequence is required for interaction with syntaxin (Hata et al. supra). Phosphorylation of Munc-18 by protein kinase C is also implicated in regulating interaction with syntaxin (Fujita, Y. et al. (1996) J. Biol. Chem. 271:7265-68).
The discovery of a new vesicle transport protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of cancer and immune disorders.